1. Field of the Invention
The present invention relates to multilayer printed wiring boards with through-holes and methods of manufacturing the same.
2. Description of the Related Art
Most electronic systems include printed wiring (or circuit) boards with high density electronic interconnections. A printed circuit board may include one or more circuit cores, substrates, or carriers. In one fabrication scheme for the printed circuit board having the one or more circuit carriers, electronic circuitries (e.g., pads, electronic interconnects, patterns, etc.) are fabricated onto opposite sides of an individual circuit carrier to form a pair of circuit layers. These circuit layer pairs of the circuit board may then be physically and electronically joined to form the printed circuit board by fabricating an adhesive (or a prepreg or a bond ply), stacking the circuit layer pairs and the adhesives in a press, curing the resulting circuit board structure, mechanically drilling (or laser drilling) through-holes, and then plating the through-holes with a copper material to interconnect the circuit layer pairs.
In some designs requiring high reliability, these printed circuit boards are formed by filling the through-holes with a conductive ink (e.g., CB100, manufactured by DuPont, Inc., or equivalent substitute from different supplier(s)) or a non-conductive ink (e.g., PHP-900, manufactured by San-Ei Kagaku Co. Ltd., or equivalent substitute from different supplier). These ink filled holes are more reliable than non-filled holes because the cured ink plug acts as support to
However, reliability problems may still occur with these printed circuit boards having the ink filled holes. These reliability problems typically occur during the component assembly process of the printed circuit boards because this is when the printed circuit boards are exposed to a series of thermal heat excursions. It is during these thermal heat excursions that a conductor on a surface (e.g., a copper plated cap) of a plated through-hole that is filled with ink materials can separate from the electrolytic copper plated hole-wall as shown by the arrows in FIGS. 1A, 1B, 2A, and 2B.
In order to help reduce this separation, IPC (Association Connecting Electronics Industries) introduced some new requirements known as “wraparound plating thickness” (IPC-6012B Amendment 1—December 2006 Qualification and Performance Specification for Rigid Printed Boards). This wraparound copper does help to reduce the occurrence of surface conductor separation from holes with barrel-plated copper as shown in FIGS. 3 and 4.
However, the wraparound plating thickness does not completely eliminate the occurrences of circuit board failures, such as surface conductor separation from holes with barrel-plated copper as shown by the arrows in FIGS. 5 and 6. At the same time, the wraparound plating thickness makes it very difficult and almost impossible to manufacture some of the designs with sequential lamination cycles (e.g., where a sub-component of a circuit board is laminated again more circuit layers or another sub-component of the circuit board) and/or designs with multiple blind via holes that start at a common conductor layer. Each wraparound process increases the surface plated copper by approximately 0.0005″. This increase in the surface plated copper reduces the space between traces, limiting the ability to produce fine line conductors on the layers with wrap plating.
Furthermore, the conductive or non-conductive ink filled holes need to be planarized (leveled) after the through-holes are filled with ink and cured. Occasionally, this planarization operation removes the wraparound copper that was previously plated. The printed circuit boards with no wraparound copper are subject to rejection as per IPC-6012B. This condition renders the printed circuit boards susceptible to have the copper plated cap separate from the hole-wall as shown in FIGS. 1 and 2. Unfortunately and as mentioned above, this separation of the cap from the hole-wall may happen during the component assembly process.